Infrared sensors have been used commonly for the purpose of detecting and tracking targets which emit an infrared signal generated by a hot body or exhaust. One characteristic of infrared sensors is that they have difficulty tracking targets when the sun is in the field of view. As the sun is a black body, it produces radiation throughout the entire infrared band. Thus, when the sun is in the field of view, the infrared sensor will sense the sun's presence. As the sun's intensity is great, it commonly damages the detector focal plane.
A guard band filter can prevent the detector processor from tracking a false target. A guard band filter is used to reject an strong signal present from the sun or a flare.
One method that has been used to prevent damage to the detector is to restrict the sensor sensitivity in a wave length band where the sun's energy is reduced. One region that is commonly used for this purpose is the ozone (O.sub.3) wave length absorption band, called the solar blind region which is between 0.265 .mu.m and 0.275 .mu.m. The ozone region is extremely useful because the sun's rays are attenuated in the ozone layer which is at an altitude between 20,000 and 40,000 feet. Thus, the sun's rays in this wave length band at the earth's surface are substantially attenuated and do not effect the detector. However, as ozone is not as prevalent in the atmosphere below 20,000 feet, the detector is still able to sense targets even at considerable distances.
Another method is to use a narrow band filter so the detector is sensitive to a specific wave band where the sun's rays are constantly attenuated yet the target signal is still present. For instance, the carbon dioxide emission bands from the IR signal of a hot aircraft jet exhaust at short range can be detected in the CO.sub.2 absorption wave band which attenuates the solar radiation. This type of a device has been utilized at high altitudes with low IR attenuation to sense signals from known emitters which gives off a strong IR signal in a specific wave band. However, the distance that the sensor is operational at sea level is short since the IR signal is strongly attenuated.